how does Fitzgerald achieve a melancholic mood in the beginning of this chapter

Gatsby's death takes place on the first day of autumn, when a chill has begun to creep into the air. His decision to use his pool is in defiance of the change of seasons, and represents yet another instance of Gatsby's unwillingness to accept the passage of time. The summer is, for him, equivalent to his reunion with Daisy; the end of the summer heralds the end of their romance.